This number is a prime.
643 = (38 + 83)/(3 + 8). [Trotter]
There is a 6-4-3 double play in baseball. [Patterson]
According to "The New Book of Prime Number Records," the product of the first 643 primes plus one is prime. This integer consists of 2038 digits and is known in MATHEMATICA as Euclid . Also note that 643 is a prime. [Schiffman]
As Alois P. Heinz discovered, 643 is the smallest nonprime among integers a(n) greater than 1 so that multiplying the n-th highly composite number by a(n) will give a highly composite number. a(643) = 6 is not prime. [Post]
The smallest zero in "Cald's Sequence" occurs at the prime number 643. [Brockhaus]
(6^6 + 4^4 + 3^3) is divisible by 643. Are there any other multi-digit primes with this property? [Gaydos]
The 643-meter-high Tokyo Skytree is one of the most iconic towers in Japan and the world. [Yuki]